1. Technical Field
The invention is related to parameter encoders of the type which classify a received RF signal in accordance with a predetermined set of parameters, such as frequency, pulse width, amplitude, time of arrival, type of modulation, and the like.
2. Background Art
Various digital RF receiver systems include a digital channelizer which divides a wide receiver frequency band into many narrow frequency "bins" or channels, so that the receiver can and digitally process each individual channel separately. Such a digital RF receiver may be employed in a digital cellular telephone system, for example. As another example, it may be employed with a parameter encoder. A parameter encoder is a device in a digital RF receiver which characterizes each received RF signal in accordance with a predetermined set of parameters, such as frequency, pulse width, amplitude, time of arrival, type of modulation, and the like, and transmits that information to a user. The user may be an electronic communication system. The division of the wide frequency band into narrow channels is best performed using uniform polyphase filter banks of the type well-known in the art. The highest RF frequency which can be processed is limited by the sampling rate capability of the polyphase filters. As is well-known, the Nyquist rate is the minimum sampling rate at which a particular RF frequency can be measured, and is equal to twice the particular RF frequency. The bank of polyphase filters must be able to run at the Nyquist rate corresponding to the highest frequency of the RF band of the receiver. Stated differently, the reason that a filter bank would be operated at this rate is that this is the slowest rate at which it can be operated to cover the receiver's RF frequency band, and determines the clock rate for which the hardware must be designed. By thus minimizing the hardware clock rate, hardware costs are minimized.
Digital receivers using uniform polyphase filter banks operating at the Nyquist rate suffer from the problem of aliasing between channels. Aliasing is a problem that occurs in all digital systems, and manifests itself by making frequencies outside the Nyquist bandwidth map to frequencies that are either higher or lower than the actual frequency of the received signal, a significant problem.
One way of eliminating aliasing between channels is to oversample the received signal and run the hardware including the polyphase filter banks at a rate exceeding the Nyquist rate. However, oversampling the signal and running the hardware at the higher rate is not desirable because it makes building the hardware for a wide frequency bandwidth (e.g., 1 GHz) costly or difficult, if not impossible with current technology.
Another way of eliminating aliasing between channels for a given RF signal bandwidth is to employ filters with narrower frequency bins (i.e., "narrower" filters) and employ a proportionately greater number of such narrower filters in the polyphase filter bank. However, narrowing the filters and increasing their number is not desirable because it increases the physical size of the hardware, and therefore increases the power consumption and heat dissipation.
The present invention is directed to eliminating or ameliorating the problem of aliasing between channels without increasing the sampling rate and without increasing the number of filters. In particular, the invention is directed to more reliable ways of processing a received signal whose apparent frequency is near a frequency boundary separating adjacent channels. More specifically, the invention is directed to a more reliable measurement of the frequency of a received signal of a frequency near the boundary between adjacent channels.